speed limits

You're cruising through Idaho at a cool 75 mph. You envision yourself in Portland an hour early. But then you cross the Oregon border: same road, lower speed limit. If you've ever applied the brakes while wondering where these seemingly arbitrary limits come from and who decides them, we've got some surprisingly interesting answers.

The origin of speed limits

The first U.S. speed limit for cars (aka horseless carriages) was set in 1901 in Connecticut. It limited drivers to 12 mph in the city and 15 in the country. (So that's why it took so long to settle out West.) New York was next, and slowly but surely all states developed their own speed limits.

The national speed limit

States had the power to establish their own speed limits until 1973, when Congress enacted the first national speed limit of 55 mph. This was partly aimed at limiting Americans' gas guzzling in the face of an oil shortage.

The 55–mph limit was raised for rural interstates in 1987, and states got back their speed-limit-setting power in 1995.

Speed limits today

Here are some commonly posted speed limits by road type:

Rural interstates

60–85 mph

Urban interstates

55–75 mph

Limited-access roads

50–75 mph

Alleys

10–15 mph

School zones (during school hours)

15–25 mph

Residential roads

25–35 mph

Though this list gives you an idea of what to look for while driving, posted limits can vary.

How do states determine speed limits?

How do states decide on their speed limits — blindfold and a dartboard? Not exactly. Here are some of the main factors at play:

The 85 percent

The "85th percentile" method sets the limit at the speed 85 percent of drivers travel — regardless of the currently posted limit. States that trust the 85th percentile as a valuable benchmark assume that most drivers are reasonable and safe and that it makes sense to base the limit on what they do. Some officials reject this method, however, arguing that the 85th percentile isn't static and actually just rises along with the posted limit.

Road characteristics

In addition to the 85th percentile rule, officials also consider several characteristics of the road, such as hills, curves, traffic density, and pedestrian use (especially school zones).

Accident history

If nothing else, speed limits are great for learning from mistakes. If officials see a sharp increase in accident rates on a stretch of road, a lower speed limit may be in order.

Speed limit trivia: an Esurance infographic

Why your car insurer cares so much about speeding

Speeding is the #3 cause of car accidents, and it's responsible for 1 out of 3 driving fatalities. It's also expensive: speeding-related accidents cost over $40 billion every year.

Now for the good news: Since speeding can be so costly, car insurers are big on drivers who adhere to the limits (however random they may seem). Keeping your driving record free from speeding tickets is one way to build trust with your insurer and keep your rates low. So it literally pays to keep an eye on your speedometer and read the signs when you cross state borders.

Related links

Speeding tickets
Find out how wiping pesky speeding tickets from your driving record can help you avoid higher car insurance premiums.